Photo-assisted nitrogen doping of II-VI semiconductor compounds during epitaxial growth using an amine

ABSTRACT

The concentration of N acceptors in an as-grown epitaxial layer of a II-VI semiconductor compound is enhanced by the use of tertiary butyl amine as the dopant carrier, and is further enhanced by the use of photo-assisted growth using illumination whose wavelength is within the range of 200-250 nm.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 91,634, filed Jul. 14, 1993 now U.S. Pat. No. 5,354,708(Attorney Docket No. PHA 21,828), and assigned to the present Assignee.The specification of application Ser. No. 91,634 is incorporated hereinby reference.

BACKGROUND OF THE INVENTION

This invention relates to the growth of epitaxial layers of II-VIsemiconductor compounds, such as ZnSe, and more particularly relates toa method of incorporating N acceptors into the II-VI lattice of suchcompounds.

Co-pending parent application Ser. No. 894,308, describes a method ofN-doping epitaxial layers of II-VI semiconductor compounds during growthby flow modulation epitaxy (FME) using NH₃ as the dopant carrier.

One problem encountered when using NH₃ as the carrier, is the tendencytoward passivation of N acceptors due to the presence of H. This is dueto the inability of NH₃ to completely dissociate into N at the growthsurface. This problem persists even if photolysis of the carrier isperformed above the growth surface, probably due to the short free meanpath between the dissociated species, giving rise to recombination ofthese species.

Another problem that arises when using NH₃ as the carrier is that ittends to react with the carriers of the cation and anion species, due toits highly basic chemical nature. This results in degradation of theII-VI compound crystal lattice, such degradation increasing withincreasing flow rate of the dopant carrier gas.

Tertiary Butyl Amine (TBNH₂) has been used as the N dopant source in theepitaxial growth of ZnSe by molecular beam epitaxy (MBE). However, theTBNH₂ was dissociated into (CH₃)C--and --NH₂ by thermal cracking attemperatures in the range of about 550 to 850 degrees C prior to beingintroduced into the growth chamber. S. Zhang and N. Kobayashi, Jpn. J.Appl. Phys., 31, L666 (June 1992). Thus, the above-described problems ofpassivation and reactivity encountered when using NH₃ would be expectedto be encountered in this technique as well.

When epi layers of ZnSe are grown by the technique of MOVPE (metalorganic vapor phase epitaxy), photo-assisted growth using above bandgapillumination (illumination whose energy is above the bandgap energy ofthe semiconductor compound at the growth temperature) has been found toenhance the growth rate of the layer, permitting lower growthtemperatures, at which the sticking coefficient of the N species isincreased. Sg. Fujita et al, Jpn. J. Appl. Phys., 26, L2000 (1987). Inaddition, the use of below-bandgap illumination has been observed toenhance doping efficiency on the Se sublattice. Sz. Fujita et al, J.Cryst. Grow., 101, 48 (1990).

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the invention to provide a method for N-dopingepitaxial layers of II-VI semiconductor compounds (such as ZnSe and itsalloys) which overcomes the above disadvantages encountered when usingNH₃ or dissociated TBNH₂ as the carrier species.

In accordance with the invention, it has been discovered that when anamine is employed as the carrier for doping N into II-VI semiconductorcompound lattices during epitaxial growth, and is introduced into theepitaxial growth chamber in an undissociated state, the N acceptorconcentration is increased.

This result is believed to be due to the fact that the C--NH₂ bondstrength in amines is weaker than the H--NH₂ bond strength in amines andammonia. As a result, a larger concentration of NH₂ at the growthsurface is expected to be available for further dissociation into N andNH. In addition, the carbon group radical is available to extract an Hfrom the --NH₂ radical as the C--NH₂ bond dissociates. The remaining Hcould be extracted by a CH₃ attached to a Zn on a neighboring latticesite.

Another advantage of amines is their larger molecular size relative toNH₃, which is expected to result in less reactivity with the othergrowth species carriers, such as MO Zn and MO Se, resulting in lessdegradation of the crystal lattice and less compensating defects(donor-like complexes involving N and native point defects).

As used herein, the term "amine" means R--NH₂, where R is a hydrocarbongroup radical and includes the homologous series (C_(x) H_(2x+1))--C_(y)--NH₂, where x is from 2y+1 to 18, and y is from 0 to 1. Typicalexamples are the primary alkyl amines, such as ethyl amine, methylamine, iso propyl amine, tertiary butyl amine and pentyl amine.

The temperature at which growth is carried out must not exceed about 500degrees C., above which thermal cracking of the amine before it reachesthe growth surface becomes likely, and the sticking co-efficient of N onthe growth surface becomes negligible. Accordingly, the growthtemperature preferably should not exceed about 400 degrees C.

In accordance with a preferred embodiment of the invention, growth iscarried out by the technique of vapor deposition, referred to variouslyin the art as CVD (chemical vapor deposition), VPE (vapor phase epitaxy)and FME (flow modulation epitaxy). Where the growth species are carriedas metal organic (MO) compounds such as dimethyl Zn or Se (Dm Zn or DmSe), then CVD and VPE are referred to as MOCVD and MOVPE, respectively.

Vapor deposition is typically carried out at a pressure of from about 1to 716 Torr, as compared to about 10⁻¹⁰ to 10⁻¹¹ Torr as the backgroundpressure for molecular beam epitaxy, and generally results in highergrowth rates, and can result in more even distribution of the growthspecies along the growth surface, due to the ability to control the flowpattern of the carrier gas.

In accordance with another preferred embodiment, the growth isphoto-assisted by above-band gap or a combination of above-andbelow-bandgap illumination. Unexpectedly, such photo-assisted growthusing above-bandgap illumination (meaning illumination whose energy isabove the bandgap energy of the II-VI compound at the growthtemperature) in the wavelength region of 200 nm to 250 nm results inenhanced N acceptor concentration in the epi layer.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in terms of the growth andevaluation of a series of epitaxial layers, with reference to thefigures, in which:

FIG. 1 is a photo-luminescence (PL) spectrum of an undoped epitaxiallayer of ZnSe grown by MOVPE at 350 degrees C. and photo-assisted usinga Hg lamp, in which intensity in arbitrary units is plotted versuswavelength in Angstroms of the PL excitation source;

FIG. 2 is a PL spectrum similar to that of FIG. 1 for a ZnSe epi layerdoped with N and using TBNH₂ as the dopant carrier gas at a flow rate ofabout 10 sccm;

FIG. 3 is a PL spectrum similar to that of FIG. 2, in which growth isphoto-assisted with a gas laser;

FIG. 4 is a PL spectrum similar to that of FIG. 3, in which the flowrate of TBNH₂ is about 25 sccm;

FIG. 5 is a PL spectrum similar to that of FIG. 2, in which the flowrate is about 5 sccm, the growth temperature is about 375° C. and growthwas photo-assisted with a Hg lamp;

FIG. 6 is a PL spectrum similar to that of FIG. 5, in which the growthtemperature is 350° C.;

FIG. 7 is a PL spectrum similar to that of FIG. 6, in which the flowrate is about 2 sccm, and growth is photo-assisted with a Xe lamp; and

FIG. 8 is a PL spectrum similar to that of FIG. 7, in which the 200-250nm portion of the spectrum of the Xe lamp is filtered out.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

PL measurements were carried out while maintaining the sample at atemperature of about 7 K. to 8 K. Excitation was achieved by aiming thebeam of an argon-ion laser at the sample surface, using the UV line at3500 Å with an intensity of 0.2 mw and a spot size of about 100 micronsdiameter, resulting in an energy density of about 2w/cm².

FIG. 1 shows the PL spectrum of an undoped epi layer of ZnSe grown at350° C., photo-assisted by a mercury lamp with optical filters resultingin illumination wavelengths in the range between about 3300 and 5800 Å,corresponding to an energy range of about 3.75 to 2.14 eV, encompassingenergies above and below the bandgap energy of about 2.45 eV (˜5000 Å).At the growth temperature, DMZn (dimethyl Zn) and DMSe (dimethyl Se)vapor flows were set at 0.32 sccm and 1.6 sccm, respectively. Since thislayer is undoped, only the Free (FX) and Donor bound (D°X) excitonicemission is observed in the PL spectrum. The Acceptor bound (A°X)excitonic emission is not observed and neither is the Donor-Acceptor(D-A) pair spectrum.

FIG. 2 shows the PL spectrum of an epi layer of ZnSe doped with N usinga flow of 10 sccm of TBNH₂ (tertiary buthyl amine). All other growthparameters were similar to those used to obtain the PL spectrum ofFIG. 1. In addition to the FX and D°X peaks, the A°X peak and the D-Apair spectrum are also seen. These A°X and D-A peaks indicate theincorporation of N acceptors into the II-VI lattice.

FIG. 3 shows the PL spectrum of an epi layer of ZnSe grown under similarconditions to those for the layer of FIG. 2, except that a 193 nmexcimer laser beam having an above bandgap energy of about 6.42 eV(˜1930 Å) was positioned parallel to the sample surface. The A°X peak isslightly enhanced relative to the FX peak, as compared to FIG. 2. Thisindicates an increase in the N acceptors due to the presence of theexcimer laser beam.

FIG. 4 shows the PL spectrum of an epi layer of ZnSe grown underconditions similar to those for FIG. 3, except that the flow of TBNH₂was increased to 25 sccm. The A°X peak is further enhanced relative tothe FX peak, as compared to FIG. 3. The D-A pair spectrum intensity isalso enhanced compared to FIG. 3. This indicates that the increased flowof the dopant results in a higher N acceptor concentration. As will beappreciated, even higher flow rates should result in p-type ZnSe. As isknown, such p-type ZnSe can be used in the fabrication of blue and bluegreen LEDs and lasers.

In FIG. 4, the ratio A°X/FX is ˜20, which is two to four times largerthan the ratio obtained using a similar flow of NH₃ (A°X/FX˜5-10). Thisindicates a higher doping efficiency of N acceptors into the as-grownlayer for TBNH₂ as compared to NH₃.

We have demonstrated the incorporation of N acceptors in to ZnSe epilayers using an amine as the N carrier and using illumination withabove- and below-bandgap wavelengths, and N acceptor concentration(measured optically by PL) in the as-grown ZnSe epi layer with TBNH₂ asthe dopant carrier higher than in the case of NH₃, for identical flowrates, indicating a higher N acceptor incorporation efficiency.

Increasing the growth temperature to 370° C. from 350° C. was found todecrease the incorporation of nitrogen acceptors, indicating theadvantage of lower growth temperatures enabled by photo-assisted growth.

As will be appreciated by those skilled in the art, the concentration ofdonor-forming impurities such as chlorine in the carrier gas should bekept as low as possible, in order to minimize incorporation into theII-VI lattice where compensation of the N acceptors would occur.

As is known, a variety of Zn and Se growth precursors could be used, inany combination, for example, Diethyl Zinc, Diethyl Selenium, DimethylZinc, Dimethyl Selenium and Hydrogen Selenide.

Bandgap energy of the II-VI compound would typically range from about3.7 eV (˜3300 Å) to 2.0 eV (˜6200 Å), for example, 2.45 eV (˜5000°A) forZnSe at a growth temperature of about 350° C. Suitable illuminationsources for above-and below-bandgap photo-assisted growth include highpressure lamps, e.g., Hg and xenon lamps (energy range about 5 eV (˜2500Å) to 1.55 eV (˜8000 Å)), while above bandgap illumination could beprovided, for example, by lasers, such as exited dimer (excimer) gaslasers (energy range about 6.42 eV to 3.53 eV).

If the II-VI epi layer is grown directly on a GaAs substrate, then abuffer layer (e.g., ˜100A in thickness) of the II-VI compounds can begrown first to facilitate the growth of the doped layer. This bufferlayer could be grown using a variety of techniques.

FIG. 5 shows the PL spectrum of an epi layer of ZnSe grown at 375° C.,photo-assisted by an unfiltered Hg lamp The layer was doped with N usingTBNH₂ at a flow of 5 sccm.

FIG. 6 shows the PL spectrum of an epi layer of ZnSe grown under thesame conditions as the layer of FIG. 5, except that the growthtemperature was 350° C.

Comparison of the A°X/FX ratios of FIGS. 5 and 6 indicate that acceptorincorporation does not increase despite the growth temperature beingdecreased from 370° C. to 350° C., which should have enhanced thesticking coefficient of the Nitrogen acceptors. However, the reduceddissociation of the Zn precursor at the lower temperature could haveeffectively lowered the Zn:Se ratio at the growth surface. This in turnwould reduce the decomposition of TBNH2.

FIG. 7 shows the PL spectrum of a ZnSe epi layer grown at 350° C., withphoto-assist from a Xe lamp, and doped with N using TBNH₂ at a flow of 2sccm.

It should be noted that the Zn:Se ratio for the layers grown using theHg lamp was twice as large as the value for the layer grown using the Xelamp. Using a higher Zn:Se ratio should enhance the acceptorincorporation due to the increased availability of sites on the Sesublattice (as is well known) and perhaps additionally due to theenhanced decomposition of TBNH₂ on a Zn rich growth surface. However,comparison of the A°X/FX ratio of FIG. 6 (Hg lamp) with that of FIG. 7(Xe lamp) indicates that the acceptor incorporation is larger in thecase of the layer grown with the Xe lamp, although the layer was grownusing a lower value of TBNH₂ flow.

The layers grown using the Xe lamp have a smaller value of FWHM of theX-Ray rocking curve, compared to the layers grown using the Hg lamp,indicating a lower defect level, resulting in a higher crystallinequality.

The Xe lamp has a higher content of spectral emission in the 200 to 250nm wavelength regime relative to the other wavelengths, compared to theHg lamp. When the total intensity of the above bandgap illumination(<550 nm) is comparable for the two lamps, the Xe lamp contributes alarger intensity in the 200 to 250 nm wavelength regime.

FIG. 8 shows the PL spectrum of a ZnSe epi layer grown under the sameconditions as the layer of FIG. 7, except that an optical filter wasused to filter UV emission in the 200-250 nm range from the output ofthe Xe lamp. Comparison of the A°X/FX ratio with that of FIG. 7indicates a dramatic decrease in the level of acceptor incorporation.

These results indicate that TBNH₂ has a higher acceptor incorporationefficiency when growth is photo-assisted using a source having spectralemission in the wavelength range of about 200-250 nm.

The invention has been described in terms of a limited number ofembodiments. Other embodiments and variations of embodiments will becomeapparent to those skilled in the art, and are intended to be encompassedwith in the scope of the appended claims.

What is claimed is:
 1. A method of incorporating N acceptors into anepitaxial layer of a II-VI semiconductor compound, said methodcomprising introducing an amine in an undissociated state into a growthchamber during a photo-assisted growth of the epitaxial layer, thegrowth being photoassisted with illumination having an energy at leastabove a bandgap energy of the semiconductor compound at the growthtemperature and having spectral emission content in a wavelength rangeof 200 nm to 250 nm.
 2. The method of claim 1 in which the illuminationis provided by a Xe lamp.
 3. The method of claim 1 in which growth isphoto-assisted with illumination having energy below the bandgap energyof the semiconductor compound in addition to the above-bandgapillumination.